This invention pertains to imaging type solar energy collectors having receivers which include secondary concentrator elements. Solar energy collectors of this type are generally comprised of a receiver which intercepts solar energy coming from a adjacent reflector surface, Fresnel lens, or the like primary concentrator. To minimize thermal losses, the absorber portion of the receiver should be as small as possible, consistent with the flow of fluid coolant therethrough. However, due to the finite angular width of the sun, imperfections in the concentrator surface, alignment errors, and tracking tolerances, the receiver can not be made arbitrarliy small, but must be made large enough to intercept the sunlight reflected from any given point on the primary concentrator. The path length of reflected light measured from points on the primary concentrator to the receiver are generally a maximum at the rim of the primary concentrator, decreasing to a minimum at the center of the primary concentrator. To accommodate the resultant spread in beam size for given angular tolerances, the absorber aperture size should be larger when viewed from the concentrator rim, than when viewed from its center.
Linear focusing receivers can comprise a secondary concentrator and an energy absorber upon which solar energy is reflected from a primary concentrator. For thermal loss considerations, a receiver for a linear focusing system must have a convection suppressing cover tube with a gap large enough to reduce the attendant air conduction losses. If an evacuated receiver tube is required, there still must be a sizeable gap between the solar energy absorber elements and the cover tube, to accommodate manufacturing tolerances and bow in the absorber tube. Accordingly, such receiver designs should provied a sizeable gap between the secondary concentrator and an absorber so as to accommodate a conduction suppressing cover tube for the receiver absorber, without allowing a loss of radiation through the gap.